The present invention relates generally to an apparatus for maintaining a heat dissipating load at a substantially constant temperature, and more particularly to such an apparatus wherein variable conductance heat pipes effectively control the radiating area of a radiator.
It is desirable in many situations to maintain a heat dissipating load at a set point temperature that can be controlled over a relatively wide range. For example, in certain spacecraft missions, such as the shuttle, there are certain experiments which must be maintained at substantially constant temperatures regardless of the temperature of the spacecraft or its external environment. In the shuttle, such instruments are required to fit within a maximum envelope of 1 m.times.1 m.times.3 m, and all of the experiments within this envelope must be maintained to a temperature of .+-.1.degree. C., over a variable set point temperature within the range of 0.degree. to 30.degree. C. A system for maintaining the instrument package to within this range must have a capability of rejecting 300 watts of heat. The stated temperature range must be maintained even though the spacecraft is in many different environments having different heating effects on the instrument envelope. For example, the spacecraft may be exposed to the sun, or it may be in the dark, or it may be re-entering the atmosphere of earth, at which time the spacecraft skin temperature increases to almost 100.degree. C.
Three thermal control concepts have been previously proposed to control the temperature of the instrument envelope. These are:
(1) a heated envelope with thermostat control,
(2) a fluid loop with radiators, and
(3) a heat pipe system.
Feasibility studies indicated that the heat pipe system appears to be the most promising approach to maintain the instrument envelope to the required temperature. It was found that the fluid loop system places an additional burden on a heat rejection system of the spacecraft. In addition, to couple the fluid loop system into the system involves the use of complex interfaces, such as heat exchangers, pumps, and rotary joints. The use of a heated canister with thermostatic control requires a significant amount of power.
Heat pipes are thermodynamic devices employing the latent heat of evaporation of a fluid and wicking system to transport large quantities of heat with small temperature differences between opposite ends of a tube. Heat applied to one end (frequently referred to as the evaporator) of the pipe vaporizes a fluid that flows to the other, cold end of the pipe. At the cold end (frequently known as the condenser) the vapor is condensed causing heat to flow out of the pipe. The condensed liquid returns to the hot end of the pipe via a wick, without the aid of gravity, by capillary forces in the wick. In outer space applications, where there is no gravity, this feature makes the heat pipe ideal for temperature control. The heat pipe containers and wicks are usually made of aluminum and stainless steel, while the vaporizable fluids are usually ammonia or freon. This basic, or conventional heat pipe is known as a fixed conductance heat pipe.
A variation of the fixed conductance heat pipe is a variable conductance heat pipe. The variable conductance heat pipe contains a non-condensable gas which occupies a portion of the cold end of the heat pipe and enables the heat flow to be modulated. In response to an increase in heat at the evaporator end of the heat pipe, the vapor pressure increases to force the noncondensible gas back into a reservoir at the condenser end of the heat pipe. The decrease in gas volume in the condenser exposes a greater portion of the condenser section of the heat pipe, so that a greater quantity of heat is removed from the total system. Conversely, when the heat at the evaporator end of the heat pipe is decreased, the gas in the reservoir extends farther into the condenser end of the heat pipe to occupy a greater portion of the condenser to make it ineffective for heat transfer. The reservoir enables the device to function as a variable heat pipe to compensate for changes in the amount of heat at the evaporator end of the heat pipe and to maintain a substantially constant temperature at the evaporator end of the heat pipe.
A modification of the variable conductance heat pipe is known as the feedback variable conductance heat pipe. In the feedback variable conductance heat pipe, a reservoir of noncondensible gas is provided at the condenser end of the heat pipe. However, a heater is placed in the gas reservoir to vary the volume of the non-condensable gas within the condenser in response to a temperature sensor at the evaporator end of the heat pipe. In response to the sensor detecting a drop in temperature, the heater is activated to cause the gas in the reservoir to expand into the condenser to decrease the area of the condenser end of the heat pipe. The decrease in area of the condenser end of the heat pipe enables the temperature in the evaporator section of the heat pipe to increase. An opposite situation occurs in response to a higher temperature being sensed in the evaporator section of the heat pipe. The feedback variable conductance heat pipe provides better control for the temperature of a load coupled to the evaporator section of the heat pipe than the conventional, variable conductance heat pipe.
It has been previously reported how fixed and variable conductance heat pipes can be utilized to control the temperature of an instrument envelope, referred to as an instrument canister, to a temperature of 20.degree. C..+-.1.degree. C. In particular, the canister sidewalls are isothermalized by a system of longitudinal and circumferential heat pipes that reject heat to a radiator via variable conductance heat pipes which effectively vary the radiating area of the radiator. The canister walls are provided with fixed conductance heat pipes, which are in heat exchange relationship with variable conductance heat pipes mounted on the radiator. The canister walls and the radiator are separated by a thermal insulating blanket, with heat exchange being provided by members at spaced locations under the blanket. A heater in the reservoir is controlled in response to either the temperature of the canister wall or the temperature of the instrument to effectively vary the radiating area. It was also previously reported that the system is able to shut down during adverse periods, such as re-entry where heat soak back occurs, by activating the reservoir heater on command and forcing gas into the heat pipe, to prevent the radiators from transferring heat from the hot walls of the spacecraft back into the instrument package.
The prior art system was found to have certain disadvantages. In particular, it was capable of operation only around a set point within the vicinity of 20.degree. C., and attempts to control the set point over a range of 0.degree. to 30.degree. C. were not possible. In addition, it was found that variable conductance heat pipe headers between the canister and radiators with isothermalizer heat pipes on the radiators had a number of disadvantages, as follows:
(1) rapid freeze-out of the fluid in the isothermalizers,
(2) poor thermal coupling between the canister and heat rejecting surfaces, an important factor when attempting to control the canister temperature to a low set point in a warm environment,
(3) poor redundancy because the header design requires one variable conductance heat pipe to operate correctly; but if redundancy is necessary, two variable conductance heat pipes are necessary, and
(4) poor ground testability because the header concept requires feeder pipes to be at right angles to the header pipes, resulting in a difficult, if not impossible configuration for ground tests in the heat pipe mode.
With regard to the poor redundancy disadvantage, it is noted that if either of the variable conductance heat pipes required for redundancy develops a small leak, resulting in a gas loss, excessive radiator area would be active and the desired control would not be achieved.